The present invention relates to a process for preparing a deashed high solid concentration coal-water slurry that is easy to handle, as liquid fuel, like heavy oil in pumping, shipment, storing and the like and can be burnt by means of a boiler burner.
It is well known to prepare a coal-water slurry by grinding coal, together with water, but it is called into question how the ash content in this coal should be treated. The coal, which is normally under the ground, contains more or less of noncombustible (ash) comprising Al.sub.2 O.sub.3, SiO.sub.2, Fe.sub.2 O.sub.3 and the like. The ash contained in the coal-water slurry brings about abrasion of the boiler walls when said slurry is burnt, and lowers the efficiency of combustion of said slurry.
Under these circumstances, in the preparation of a high solid concentration coal-water slurry using a parent coal having a high ash content, there has hitherto been employed a process which comprises subjecting a relatively coarse grain-sized parent coal to gravity classification to thereby obtain a low ash coal, and grinding said low ash coal alone as the slurry material. However, this process has included the problem to be solved that no other coal than the low ash coal can be used as the slurry material, whereby the efficiency of utilization of the coal is low.
In the preparation of a high solid concentration coal-water slurry using a parent coal having a relatively high ash content, furthermore, there has hitherto been proposed a process which comprises grinding said coal and then subjecting its whole the amount of the coal to a deashing treatment, thereby lowering the ash content. However, employment of this process has also included the problems that the deashing equipment is large-sized, and accordingly not only the costs for treatment but also loss of the coal in the deashing treatment are increased.
U.S. Pat. No. 4,132,365 makes obvious a process for preparing a coal-water slurry in which particulate coal is classified and is separated into a plurality of fractions on the basis of the specific gravity thereof. Each fraction is dried and then ground, and then the fraction are mixed. In order to minimize sedimentation of the particles when they are dispersed in the aqueous medium and stabilize the slurry, it is necessary for this patent to grind the fraction having a high specific gravity more finely than the fraction having a lower specific gravity, whereby to retard the sedimentation of the fraction having the large specific gravity when dispered in water.
Some of the present inventors and others have developed a process for preparing a deashed high solid concentration slurry which exhibits a high coal recovery and a high profitability, and have already filed a patent application as U.S. patent application Ser. No. 611,069 now U.S. Pat. No. 4,593,859.
As seen from the block diagram of FIG. 2, this is a process for preparing a deashed high solid concentration slurry containing 60 wt. % or more of coal solids which comprises classifying a previously crushed parent coal into a fine-grained coal and a coarse-grained coal by means of a screen 41; feeding said coarse-grained coal to a gravity separator 42 for classifying it into a low ash coal fraction, a middle ash coal fraction and a high ash coal fraction (refuse); wet-grinding this middle ash coal fraction together with said fine-grained coal by means of a grinding mill 43 to obtain a relatively low solid concentration coal-water slurry; thereafter introducing this slurry into a flotation machine 44 for deash treatment to thereby obtain a deashed slurry (froth); introducing this froth into a dewaterer 45 to thereby obtain a relatively high solid concentration deashed cake mixing this deashed cake with said low ash coal fraction; and wet grinding this mixture by means of a grinding mill 46.
Viewed from the point of grinding the coal, this process illustrated in FIG. 2 adopts a two-stage grinding method which comprises wet-grinding a middle ash coal fraction to obtain a relatively low solid concentration first slurry; adding a coarsely ground low ash coal to this; and wet-grinding this mixture again to thereby obtain a high solid concentration second slurry. Our inventors have found that this wet type two-stage grinding method can obtain preferably particle size distribution of the second slurry by setting the solid concentration of the first slurry in the range of 40-60 wt. % and thus making it easy to control the grain size distribution of the coal in the second slurry. Accordingly, it becomes possible to prepare a deashed high solid concentration coal-water slurry containing about 70 wt. % of coal solids by incorporating both gravity classification and flotation in the wet type two-stage grinding method.
The solid concentration of a coal-water slurry is normally determined by the way in which the sturry will be used. However, the process disclosed in U.S. Pat. No. 4,593,859 is disadvantageous in that all the low ash coal obtained in the gravity classification step is mixed with the first slurry and presented to the final second wet grinding step, and so when the solid concentration of the first slurry is maintained in the range of 40-60 wt. % suitable for wet type two-stage grinding, the solid concentration of the second slurry, namely the finally obtained coal-water slurry varies according to the amount of low ash coal mixed in the first slurry. In other words, the process disclosed in U.S. Pat. No. 4,593,859 is unable to adjust the solid concentration of the coal-water slurry, namely the final product, optionally to a set value determined depending on the way in which the slurry will be used.